1. Field of the Invention
This invention relates to a multiple beam antenna and to a cellular communications network comprising such a multiple beam antenna as well as to a base station transceiver comprising such a multiple beam antenna.
2. Description of the Prior Art
The term xe2x80x9ccellular communications networkxe2x80x9d is used to refer to a communications network which is divided into geographical cells. For example, wireless communications networks are usually divided into cells which are geographical areas each of which contains a base station transceiver (BST) Mobile stations located in the communications network communicate with one or more base station transceivers, for example, the closest base station transceiver. Each base station transceiver has a limited range and a cell can be thought of as a geographical region over which a base station transceiver is intended to operate effectively.
Mobile stations such as mobile telephones may be located within a cellular communications network and send and receive signals to and from the base station transceivers. Each mobile station (not shown) operating within a cell requires a certain amount of bandwidth to operate and because the total bandwidth of base station transceivers is limited the number of mobile stations which can operate within a cell is limited.
In general an object of cellular radio communications system design is to reduce the number of cell sites required by increasing their range and or capacity. The term, xe2x80x9ccapacityxe2x80x9d is used herein to refer to any suitable measure which provides an indication of how many conventional mobile stations or other terminals are able to communicate effectively with a given antenna arrangement. Cell sites are expensive, both in terms of the equipment required and the need for a geographical site for each cell site. Geographical sites are costly and require extensive effort to obtain planning permission. In some areas, suitable geographical sites are not available.
When a cellular radio system is set up in an area of high demand, such as a city, then cell site communications capacity, rather than range, usually limits cell size. An increased cell site capacity would therefore reduce the required number of cell cites and so reduce costs.
Another aim in cellular communications network design is flexibility. That is, allowance needs to be made for changes in demand by users of the network. Previously, increases in demand have been dealt with by for example, installing new base station transceivers and splitting cells. This increases the number of cells but is expensive and time consuming and if the geographical pattern of demand changes over time the newly installed base station transceivers may later become redundant.
Another problem is that interference can occur, for example, if a signal is received by a base station transceiver, it could have been received from a mobile station within the base station transceiver""s cell or from a mobile station or base station transceiver within an adjacent or nearby cell.
One way of reducing such interference is to use two or more directional antennas at a cell site, instead of a single omni-directional antenna. This acts to reduce interference because signals received by a directional antenna at a base station transceiver have to be received from a particular direction and the likelihood of signals being received from that direction from adjacent or nearby cells is reduced. Each directional antenna can be thought of as operating effectively. over part of a cell and this part is termed a xe2x80x9csectorxe2x80x9d. By adjusting the number of sectors in a cell, the capacity of that cell can be adjusted without the need for cell splitting. For example three directional antennas have previously been used to create a tri-sectored cell. In this way the capacity of each cell is increased relative to a cell using a single omni-directional antenna because interference is reduced as described above.
Another problem for cellular or sectored communications systems in general is that so called xe2x80x9chandoffxe2x80x9d is required and this takes up valuable processing capacity within the base station transceiver and mobile stations. As a mobile station moves from one cell to another in a cellular communications system, transfer of the communication link from a first base station transceiver in the first cell to a second base station transceiver in the second cell is required and takes place by a process termed xe2x80x9chandoffxe2x80x9d. In a sectored cell, a mobile station may also move from one sector to another necessitating additional handoffs between the antenna beams of each sector within the cell. As the number of sectors increases, so does the number of handoffs and this makes increasing demands on the processing and communications capacity of the communications network.
Sectorisation is particularly advantageous in spread spectrum communications systems, such as code division multiple access (CDMA) communications systems. More detail about CDMA systems is given below. In such systems, base station transmissions, both in different sectors and in different cells, are typically in the same frequency band. Because these transmissions are in the same frequency band interference between them can be a particular problem and sectorisation is advantageous in reducing this. Another feature of spread spectrum communications systems such as CDMA and UMTS communications systems, relates to antenna beam overlap. If there is a high degree of overlap between antenna beams operating in the same frequency band (for example, in a CDMA or UMTS system) then the risk of interference between those beams increases. In order to reduce this interference, the antenna beams are separated as far as possible, but not so far as to leave large regions of the cell uncovered by any antenna beam. Mobile stations within such xe2x80x9cuncoveredxe2x80x9d regions are unable to communicate effectively with a base station transceiver or require larger amounts of power in order to do so.
Another problem relates to the high cost of electronics and cables used within base station transceivers.
It is accordingly an object of the present invention to provide a multiple beam antenna which overcomes or at least mitigates one or more of the problems noted above.
Further benefits and advantages of the invention will become apparent from a consideration of the following detailed description given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention.
According to a first aspect of the present invention there is provided a multiple beam antenna arrangement suitable for use in a cell of a cellular communications network, said cell being divided into a plurality of regions, and said arrangement being arranged to provide a specified capacity for communication with terminals located in a first one of said regions, said arrangement comprising:
an apparatus arranged to provide two or more directional antenna beams per region; and wherein said apparatus is further arranged such that in use said capacity is greater than the capacity provided to a region of a corresponding multiple beam antenna arrangement which provides only one antenna beam per region by a factor of approximately the number of antenna beams provided for said first region.
A corresponding cellular communications network is also provided comprising a plurality of cells, and wherein a plurality of said cells each contain a multiple beam antenna arrangement, said multiple beam antenna arrangement being suitable for use in a cell of a cellular communications network, said cell being divided into a plurality of regions, and said arrangement being arranged to provide a specified capacity for communication with terminals located in a first one of said regions, said arrangement comprising:
an apparatus arranged to provide two or more directional antenna beams per region; and wherein said apparatus is further arranged such that in use said capacity is greater than the capacity provided to a region of a corresponding multiple beam antenna arrangement which provides only one antenna beam per region by a factor of approximately the number of antenna beams provided for said first region.
A corresponding base station transceiver is also provided comprising: a multiple beam antenna arrangement suitable for use in a cell of a cellular communications network, said cell being divided into a plurality of regions, and said arrangement being arranged to provide a specified capacity for communication with terminals located in a first one of said regions, said arrangement comprising:
an apparatus arranged to provide two or more directional antenna beams per region; and wherein said apparatus is further arranged such that in use said capacity is greater than the capacity provided to a region of a corresponding multiple beam antenna arrangement which provides only one antenna beam per region by a factor of approximately the number of antenna beams provided for said first region.
This provides the advantage that the capacity of a cell in a cellular communications network is increased in an effective manner thus enabling costs to be reduced. Also, the multiple beam antenna arrangement may be used at existing cell sites in order to increase capacity without the need to find new cell sites.
Preferably said apparatus that is arranged to provide two or more directional antenna beams per region is arranged to provide three or more directional antenna beams per region and comprises six antenna elements. This provides the advantage that a good interleaving pattern is obtained between adjacent cells. The good interleaving allows the antenna beams to be provided with a low degree of overlap and this helps to reduce interference and improve performance and capacity.
Preferably, the antenna arrangement is suitable for use in a cell of a cellular, spread spectrum, communications network. This provides the advantage that capacity is increased effectively in a spread spectrum communications network where interference is a particular problem.
Furthermore, it is preferred that the number of cables is equal to the number of antenna beams that said apparatus is arranged to provide. This provides the advantage that costs are reduced and installation and maintenance of the antenna arrangement is simplified because, for example, in the case that three antenna beams are provided using six antenna elements, only three cables are required instead of six, one per element.
Advantageously, the antenna arrangement comprises at least one power amplifier that is arranged to be shared in use, across the antenna beams associated with a given region. This enables costs to be significantly reduced because the number of power amplifiers is reduced by allowing them to be shared. Also, greater flexibility is provided. This is especially useful in spread spectrum communications systems where power amplifiers are complex and expensive.
Preferably the antenna arrangement incorporates an amplitude taper arrangement such that in use, sidelobe levels are reduced. This provides the advantage that capacity is increased because lower sidelobe levels reduce interference.
Advantageously, said apparatus, that is arranged to provide two or more directional antenna beams, is further arranged such that said directional antenna beams are orthogonal in use. This provides the advantage that a loss less multiple beamformer may be used which improves performance of the antenna arrangement and increases capacity.